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Abstract:

A method in a mobile station for ranking cells in order to perform
reselection from a serving cell to a second cell is disclosed. The method
includes detecting a second cell that the mobile station is not allowed
to access, determining resources used for transmission of a signal of the
second cell, and applying a reselection bias if the resources used for
transmission of the signal of the second cell do not substantially
overlap resources used for transmission of a signal of the serving cell.

Claims:

1. A mobile station comprising: a wireless transceiver coupled to a
processor, the processor configured to detect a second cell that the
mobile station is not allowed to access, the processor configured to
determine resources used for transmission of a signal of the second cell,
and the processor configured to apply a reselection bias if the resources
used for transmission of the signal of the second cell do not
substantially overlap resources used for transmission of a signal of a
serving cell.

2. The mobile station according to claim 1, wherein the reselection bias
is a bias applied to the serving cell.

3. The mobile station according to claim 1, the processor configured to
apply a positive reselection bias if the resources used for transmission
of the signal of the second cell do not substantially overlap the
resources used for transmission of a signal of the serving cell, and the
processor further configured to apply a negative reselection bias if the
resources used for transmission of the signal of the second cell
substantially overlap the resources used for transmission of a signal of
the serving cell.

4. The mobile station according to claim 1, wherein the signal of the
serving cell is a reference signal of the serving cell and the signal of
the second cell is a reference signal of the second cell.

5. The mobile station according to claim 1, wherein determining the
resources used for transmission of the signal of the second cell includes
determining the resources used for transmission of the signal based on
least one of: a cell identifier of the second cell or a number of
antennas of the second cell.

6. A method in a mobile station for ranking cells in order to perform
reselection from a serving cell, the method comprising: detecting a
second cell that the mobile station is not allowed to access; determining
resources used for transmission of a signal of the second cell; and
applying a reselection bias if the resources used for transmission of the
signal of the second cell do not substantially overlap resources used for
transmission of a signal of the serving cell.

7. The method according to claim 6, applying the reselection bias to the
operating frequency of the serving cell.

8. The method according to claim 6 further comprising applying a positive
reselection bias if the resources used for transmission of the signal of
the second cell do not substantially overlap the resources used for
transmission of a signal of the serving cell, and applying a negative
reselection bias if the resources used for transmission of the signal of
the second cell substantially overlap the resources used for transmission
of a signal of the serving cell.

9. The method according to claim 6, wherein determining the resources
used for transmission of the signal of the second cell includes
determining the resources used for transmission of the signal based on
least one of: a cell identifier of the second cell or a number of
antennas of the second cell.

10. A mobile station comprising: a wireless transceiver coupled to a
processor, the processor configured to determine that time frequency
resources used for transmission of a signal from a first cell, with which
the mobile station is associated on a first frequency, substantially
overlap time frequency resources used for transmission of a signal from a
second cell operating on the first frequency, and the processor
configured to associate the mobile station with a third cell operating on
a second frequency.

11. The mobile station according to claim 10, the processor configured to
associate the mobile station with the third cell operating on the second
frequency by reselecting to the third cell operating on the second
frequency.

12. The mobile station according to claim 10, the processor configured to
associate the mobile station with the third cell operating on the second
frequency if a signal level of the first cell is below a first threshold.

13. The mobile station according to claim 12, wherein the first threshold
is signaled to the wireless terminal.

14. The mobile station according to claim 10 the processor configured to
determine that the time frequency resources used for transmission of the
signal from the first cell do not substantially overlap the time
frequency resources used for transmission of the signal from the second
cell operating on the first frequency; and the processor configured to
associate the mobile station with the third cell operating on the second
frequency if a signal level of the first cell is below a second
threshold.

15. The mobile station according to claim 10, wherein the signal of the
first cell is a reference signal and the signal of the second cell is a
reference signal.

Description:

CROSS-REFERENCE TO RELATED APPLLICATIONS

[0001] The present application claims benefits to provisional Application
No. 61/421,377 filed on 10 Nov. 2010, the contents of which are
incorporated herein by reference.

[0003] Wireless communication networks are well known. Some networks are
completely proprietary, while others are subject to one or more standards
to allow various vendors to manufacture equipment for a common system.
One such standards-based network is the Universal Mobile
Telecommunications System (UMTS). UMTS is standardized by the Third
Generation Partnership Project (3GPP), a collaboration between groups of
telecommunications associations to make a globally applicable third
generation (3G) mobile phone system specification within the scope of the
International Mobile Telecommunications-2000 project of the International
Telecommunication Union (ITU). Efforts are currently underway to develop
an evolved UMTS standard, which is typically referred to as UMTS Long
Term Evolution (E-UTRA) or Evolved UMTS Terrestrial Radio Access
(E-UTRA).

[0004] According to Release 8 of the E-UTRA or LTE standard or
specification, downlink communications from a base station (referred to
as an "enhanced Node-B" or simply "eNB") to a wireless communication
device (referred to as "user equipment" or "UE") utilize orthogonal
frequency division multiplexing (OFDM). In OFDM, orthogonal subcarriers
are modulated with a digital stream, which may include data, control
information, or other information, so as to form a set of OFDM symbols.
The subcarriers may be contiguous or discontiguous and the downlink data
modulation may be performed using quadrature phase shift-keying (QPSK),
16-ary quadrature amplitude modulation (16QAM), or 64QAM. The OFDM
symbols are configured into a downlink subframe for transmission from the
base station. Each OFDM symbol has a time duration and is associated with
a cyclic prefix (CP). A cyclic prefix is essentially a guard period
between successive OFDM symbols in a subframe. According to the E-UTRA
specification, a normal cyclic prefix is about five (5) microseconds and
an extended cyclic prefix is 16.67 microseconds.

[0005] In contrast to the downlink, uplink communications from the UE to
the eNB utilize single-carrier frequency division multiple access
(SC-FDMA) according to the E-UTRA standard. In SC-FDMA, block
transmission of QAM data symbols is performed by first discrete Fourier
transform (DFT)-spreading (or precoding) followed by subcarrier mapping
to a conventional OFDM modulator. The use of DFT precoding allows a
moderate cubic metric/peak-to-average power ratio (PAPR) leading to
reduced cost, size and power consumption of the UE power amplifier. In
accordance with SC-FDMA, each subcarrier used for uplink transmission
includes information for all the transmitted modulated signals, with the
input data stream being spread over them. The data transmission in the
uplink is controlled by the eNB, involving transmission of scheduling
requests (and scheduling information) sent via downlink control channels.
Scheduling grants for uplink transmissions are provided by the eNB on the
downlink and include, among other things, a resource allocation (e.g., a
resource block size per one millisecond (ms) interval) and an
identification of the modulation to be used for the uplink transmissions.
With the addition of higher-order modulation and adaptive modulation and
coding (AMC), large spectral efficiency is possible by scheduling users
with favorable channel conditions.

[0006] E-UTRA systems also facilitate the use of multiple input and
multiple output (MIMO) antenna systems on the downlink to increase
capacity. As is known, MIMO antenna systems are employed at the eNB
through use of multiple transmit antennas and at the UE through use of
multiple receive antennas. A UE may rely on a pilot or reference symbol
(RS) sent from the eNB for channel estimation, subsequent data
demodulation, and link quality measurement for reporting. The link
quality measurements for feedback may include such spatial parameters as
rank indicator, or the number of data streams sent on the same resources;
precoding matrix index (PMI); rank indicator (RI) and coding parameters,
such as a modulation and coding scheme (MCS) or a channel quality
indicator (CQI). Together MCS or CQI, PMI and RI constitute elements of
the Channel State Information (CSI) which convey the quality of MIMO
channel indicative of the reliability and condition number of the channel
capable of supporting multi-stream communication between the eNB and the
UE. For example, if a UE determines that the link can support a rank
greater than one, it may report multiple CQI values (e.g., two CQI values
when rank=2 by signaling of the corresponding RI). Further, the link
quality measurements may be reported on a periodic or aperiodic basis, as
instructed by an eNB, in one of the supported feedback modes. The reports
may include wideband or subband frequency selective information of the
parameters. The eNB may use the rank information, the CQI, and other
parameters, such as uplink quality information, to serve the UE on the
uplink and downlink channels.

[0007] E-UTRA systems must be compliant to regulatory requirements on
spurious emissions on licensed bands in different regions of the world.
E-UTRA follows the "uplink after downlink" principle which means that a
UE must transmit on its uplink only when its downlink is reliable. In
other words, a UE that does not have a reliable downlink must
continuously monitor the quality of the downlink signal by tracking the
downlink signal quality (e.g., based on channel state estimation) and
stop transmission on its uplink if the downlink signal quality falls
below a threshold. In E-UTRA, this is enabled by means of Radio Link
Monitoring (RLM) UE procedures where a UE continuous monitors the
cell-specific reference signal (CRS) on the downlink and determines the
channel state (including estimating the propagation channel between the
eNB and the UE and the underlying interference on the same carrier). Qout
is defined as the condition that the channel quality between eNB and the
UE is such that the Block Error Rate (BLER) of a first hypothetical
control channel transmission exceeds 10%. This event is also denoted as
an "out-of-sync" event. Qin is defined as the condition that the channel
quality between eNB and the UE is such that the BLER of a second
hypothetical control channel transmission drops below 2%. This event is
also denoted as an "in-sync" event. The UE monitors the channel state in
RRC_CONNECTED mode continuously or periodically in both non-discontinuous
reception (non-DRX) and discontinuous reception (DRX) states to evaluate
whether Qout or Qin has occurred. Upon several successive Qout
detections, the UE must determine that a Radio Link Problem (RLP) has
occurred. In the RLP state, the UE must assume that it has lost its
downlink with the serving eNB and start monitoring the link for recovery.
If a Qin is detected within a certain duration of time as configured by
the eNB by means of a Radio Resource Control (RRC) timer, the UE resumes
normal RRC_CONNECTED operation. On the other hand, if a Qin is not
detected within the said duration of time, the UE must determine that a
Radio Link Failure (RLF) has occurred and must stop all uplink
transmission within 40 ms. The RLM procedure reduces the probability that
a UE jams the uplink of a neighbor cell when the UE has lost the serving
cell downlink but has not been handed over to a different cell by the
network due to Radio Resource Management (RRM) inefficiencies.

[0008] Like other 3GPP standards, E-UTRA supports mobility of UEs by RRM
measurements and associated support for RRC signaling including specified
eNB and UE behavior in both RRC_CONNECTED and RRC_IDLE states. In the
RRC_CONNECTED state, a UE can be configured to measure and report
Reference Signal Received Power (RSRP) and Reference Signal Received
Quality (RSRQ) for both the serving cell and the neighbor cells (on the
serving cell carrier and inter-frequency carriers). A network element
such as the eNB or the Mobility Management Entity (MME) can perform UE
handovers based on the reported measurements. In RRC_IDLE state, the UE
can be configured to measure RSRP and RSRQ and perform cell reselections
based on these measurements.

[0009] Heterogeneous networks comprise a variety of base stations serving
mobile stations. The base stations can operate on the same carrier
frequency. The variety of base stations can include some or all of the
following types of base stations: conventional macro base stations (also
referred to as macro cells), pico base station (or pico cells), relay
nodes and femto base stations (also referred to as femto cells, CSG cells
or Home eNodeBs). Macro cells typically have coverage areas that range
from several hundreds of meters to several kilometers. Pico cells, relays
and femto cells can have coverage areas that are considerably smaller
than the coverage area of typical macro cells. Pico cells can have
coverage areas of about 100-200 meters. Femto cells are typically used
for indoor coverage, and can have coverage areas in the 10s of meters.
Relay nodes are characterized by a wireless backhaul to a donor base
station, and can have coverage areas similar to pico cells.

[0010] Heterogeneous networks can potentially enable an operator to
provide improved service to users (e.g., increased data rates, faster
access, etc) with lower capital expenditure. Typically, installation of
macro base stations is very expensive as they require towers. On the
other hand base stations with smaller coverage areas are generally much
less expensive to install. For example, pico base stations can be
installed on roof tops and femto base stations can be easily installed
indoors. The pico and femto base stations allow the network to offload
user communication traffic from the macro cell to the pico or femto
cells. This can enable the users to get higher throughput and better
service without the network operator installing additional macro base
stations or provisioning more carrier frequencies for communication.
Thus, heterogeneous networks are considered to be an attractive path for
evolution of wireless communication networks. 3GPP has commenced work on
enabling heterogeneous LTE networks in 3GPP LTE Release 10.

[0011] Currently, the existing Rel-8/9 UE measurement framework can be
made use of to identify the situation when this interference might occur
and the network can handover the UE to an inter-frequency carrier which
is not shared between macro-cells and HeNBs to mitigate this problem.
However, there might not be any such carriers available in certain
networks to handover the UE to. Further, as the penetration of HeNBs
increases, being able to efficiently operate HeNBs on the entire
available spectrum might be desirable for maximizing spectral efficiency
and reducing overall operational cost. Several other scenarios are likely
too including the case of a UE connected one HeNB experiencing
interference from an adjacent HeNB or a macro cell. The following types
of interference scenarios have been identified.

[0012] HeNB (aggressor)→MeNB (victim) downlink (DL)

[0013] HUE (aggressor)→MeNB (victim) uplink (UL)

[0014] MUE (aggressor)→HeNB (victim) UL

[0015] MeNB (aggressor)→HeNB (victim) DL

[0016] HeNB (aggressor)→HeNB (victim) on DL

[0017] HeNB (aggressor)→HeNB (victim) on UL.

[0018] FIG. 1 illustrates an LTE Heterogeneous network comprising a macro
cell, pico cells and femto cells operating on a single carrier frequency.
A mobile station (also referred to as "user equipment" or `UE") may be
associated with one of the cells based on its location. The association
of a UE to a cell can refer to association in idle mode or connected
mode. That is, a UE is considered to be associated with a cell in idle
mode if it is camped on the cell in idle mode. Similarly, a UE is
considered to be associated with a cell in connected mode if it is
configured to perform bi-directional communication with a cell (for
example, a UE in LTE RRC connected mode can be connected to, and
therefore associated with a cell). A UE associated with a macro cell is
referred to macro UE; a UE associated with a pico cell is referred to as
a pico UE; and a UE associated with a femto cell is referred to as a
femto UE.

[0019] Various time-division approaches are possible for ensuring that the
base stations in a heterogeneous network share the frequency spectrum
while minimizing interference. Two approaches can be envisioned:

[0020] A network can configure time periods where different base stations
are required to not transmit. This enables cells that can interfere with
one another to transmit in mutually exclusive time periods. For example,
a femto cell can be configured with some time periods during which it
does not transmit. If a macro UE is located within the coverage of the
femto cell, the macro cell can use the time periods during which the
femto cell does not transmit to transmit data to the UE.

[0021] The network can configure time periods where a first base station
transmits on all available time periods (e.g., pico eNBs), while a second
base station (e.g., macro eNB) transmits only on subset of the available
time periods. A UE connected to the first base station can therefore have
two "virtual" channels at different channel qualities depending on how
much the second base station's transmission interferences with that for
the first (i.e., signal geometry of the first base station relative to
the second). The first virtual channel is where only the first base
station transmits data while the second base station does not transmit
data. The second virtual channel is one where both the first and the
second base stations transmit data. The first base station can use
adaptive modulation and coding and schedule at different MCS levels on
the two virtual channels (in the extreme case, not schedule at all on the
second virtual channel when the interference from the second base station
is large.)

[0022] However, it should be noted that the time division approaches can
lead to various problems for UEs in idle mode, some of which are listed
below:

[0023] A UE in idle mode expects to receive paging messages from a serving
cell in certain predefined time periods that occur periodically. When the
paging time periods overlap the time periods when a strong neighbor cell
transmits data, the UE may be unable to receive paging messages.

[0024] The cell specific reference symbol (CRS) transmissions of the
serving cell may overlap the CRS of a strong neighbor cell. This can
result in the UE being unable to perform correct measurements of the
serving cell and the neighbor cell.

[0025] The physical broadcast channel (PBCH) transmission of the serving
cell may overlap the PBCH transmission of a strong neighbor cell,
resulting in the UE being unable to decode the PBCH of the serving cell.
This can the result in the UE not having up to date system information of
the serving cell, as well as other undesirable consequences.

[0026] The primary synchronization signal (PSS) and secondary
synchronization signal (SSS) of the serving cell may overlap the PSS and
the SSS of a strong neighbor cell respectively. This can result in the UE
not being able to remain synchronized to the serving cell.

[0027] Therefore, methods to overcome the problems in idle mode UEs
resulting from the use of time division approaches are needed.

[0034] FIG. 6B is a paging offset determination process in a base station.

[0035] FIG. 6C is a paging offset determination process in a UE.

[0036]FIG. 7A illustrates a first embodiment that overcomes problems
related to overlap or collision of cell-specific reference symbols of
different cells in a heterogeneous network.

[0037] FIG. 7B illustrates a second embodiment that overcomes problems
related to overlap or collision of cell-specific reference symbols of
different cells in a heterogeneous network.

[0038]FIG. 8A illustrates a first embodiment that overcomes problems
related to overlap or collision of physical broadcast channels of
different cells in a heterogeneous network.

[0039] FIG. 8B illustrates a second embodiment that overcomes problems
related to overlap or collision of physical broadcast channels of
different cells in a heterogeneous network from the UE perspective.

[0040]FIG. 8c illustrates a second embodiment that overcomes problems
related to overlap or collision of physical broadcast channels of
different cells in a heterogeneous network from the femto cell
perspective.

DETAILED DESCRIPTION

[0041] Femto cells are generally used in homes and offices and their
precise location and configuration is not entirely under the network
operator's control. For example, two femto cells located in nearby homes
can have the same physical layer cell identifier (PCID). A femto cell can
be a restricted access cell such as a Closed Subscriber Group (CSG) cell.
FIG. 1 illustrates an example of Heterogeneous network (100) comprising a
macro cell (102), femto cells (104, 108, 122), pico cells (112, 124) and
mobile stations (106, 110, 116, 118, 120, 126). If a UE (110) is not a
member of the CSG to which the femto cell (108) belongs, the UE may be
unable to access the femto cell. Even if the UE (110) is very close to
such a femto cell (108), the UE may be associated with the macro cell.
The UE may then experience significant interference to its communication
with the macro cell due to transmissions of the femto cell.

[0042] Pico cells generally do not restrict access to specific users.
However, some operator configurations can allow pico cells to restrict
access to certain users. Pico cells are generally fully under the network
operator's control and can be used to enhance coverage in locations where
the macro cell signal may be inadequate. Furthermore, in order to
maximize offloading of users to pico cells, a network operator can have
an association bias towards the pico cell. That is, a UE (118) is made to
associate with a pico cell even if the pico cell (112) is not the
strongest cell at the UE's (118) location. This is referred to as "Cell
range expansion" of the pico cell. A UE is said to be the cell range
expansion area of a pico cell, if it associates with the pico cell only
if an association bias is used, and associates with another cell (e.g., a
macro cell 102) if the association bias is not used. If a UE (118) is in
the cell range expansion area of the pico cell (112) and is associated
with the pico cell (112), it can experience significant interference due
to transmissions of a neighbor cell (such as a macro cell 102).

[0043] In order to operate multiple cells with overlapping coverage on a
carrier frequency, such as in a heterogeneous network 100, it is
necessary to have coordination between the cells so that the
transmissions don't interfere with one another. LTE heterogeneous
networks will use time division techniques to minimize interference.
Specifically, a cell can be configured with patterns of subframes during
which it does not schedule user data. Such subframes are referred to as
"Blank subframes". Furthermore, it may be necessary to transmit some
critically important information in all subframes. For example, it may be
necessary to transmit cell-specific reference symbols (CRS) to enable UEs
to perform measurements during the subframe. It may also be necessary to
transmit primary and secondary synchronization signals (PSS and SSS),
primary broadcast channel (PBCH) and System Information Block 1 (SIB1),
Paging Channel and the Positioning Reference Signal (PRS). Such
information is essential for proper operation of functions such as cell
search and maintenance of up-to-date system information. Blank subframes
which are not used for scheduling data but can be used for transmission
of a restricted set of information (such as the critically important
information described above) are referred to as "Almost blank subframes"
(AB subframes). In LTE AB subframes of a base station, the base station
can be configured to not transmit any energy on all resource elements,
except for resource elements used for (a) CRS, (b) PSS and SSS, (c) PBCH,
(d) SIB1, e) paging messages, and (e) Positioning Reference Signal (PRS).
There may be other signals such as Channel State Information Reference
Signal (CSI-RS) in the AB subframes.

[0044] AB subframes of one cell can be used by a neighboring cell to
schedule UEs. FIG. 2 illustrates the use of AB subframes. For example,
each of a femto cell, a macro cell and a pico cell can be configured with
an AB subframe pattern. The patterns can be such that the AB subframes of
different cells can overlap. Alternatively the patterns can be mutually
exclusive, so that AB subframes of two cells do not overlap. Also, some
cells may not be configured with an AB subframe pattern. As indicated
above, a cell can be configured to only transmit critically important
information during its AB subframes.

[0045] We further illustrate the use of AB subframe patterns. A macro UE
may be in the coverage of a non-allowed femto cell (such as a CSG cell
whose CSG the UE is not a member). UE 110 represents such a UE and femto
cell 108 represents such a femto cell. Such a macro UE can experience
very high interference from the femto cell, making communication between
the macro UE and the macro cell very difficult. To overcome the
interference, the macro cell can transmit data to the UE only in the AB
subframes of the femto cell. Since the femto cell only transmits
critically important signals in the AB subframes, the macro cell can
avoid most of the interference from the femto cell and successfully
transmit data to the macro UE in the AB subframes of the femto cell.

[0046] Similarly, a pico UE may be in the cell range expansion area of the
pico cell. UE 118 represents such a pico UE and pico cell 112 represents
such a pico cell. Such a pico UE can experience a very high interference
from a neighbor cell (such as macro cell 102), making communication
between the pico UE and the pico cell very difficult. In order to
overcome the interference, the pico cell can transmit data to the UE only
in the AB subframes of the macro cell. Since the macro cell only
transmits critically important signals in the AB subframes, the pico cell
can avoid most of the interference from the macro cell and successfully
transmit data to the pico UE in the AB subframes of the macro cell.

[0047] When different cells use different patterns of AB subframes, the
RRM, RLM and CSI measurements performed by UEs in the heterogeneous
network can result in unpredictable and undesirable behavior. UEs perform
RLM measurements in connected mode to ensure that the serving cell signal
conditions are adequate to schedule the UE. UEs perform RRM measurements
to support handovers in connected mode and reselections in idle mode.
Furthermore, UEs can perform RRM measurements in idle mode to support
idle mode mobility (i.e., cell selection and cell reselection). UE
performs CSI measurements to support optimal scheduling by the base
station. For example, macro UE 110 in the coverage of a non-allowed femto
cell 108 may be performing RLM measurements of the macro cell 102 signal.
Due to interference from the femto cell 108 in subframes during which the
femto cell schedules (i.e., not the AB subframes of the femto cell), the
macro UE can conclude that the radio link between the macro cell and the
macro UE has failed. The UE can make such a conclusion even if it can be
successfully scheduled by the macro cell during the AB subframes of the
femto cell.

[0048] Similarly, the macro UE 110 in the coverage of a non-allowed femto
cell 108 may be performing RRM measurements of the serving cell and
neighbor cells. Due to interference from the femto cell, the UE may
measure a low value the macro cell signal level and transmit a
measurement report indicating the low value to the network. As a result
of the measurement report, the network can perform a handover of the UE
to another frequency or to another radio access technology (such as UMTS
or GSM). This is an undesirable outcome, as the UE can be successfully
scheduled by the macro cell in the femto cell's AB subframes.
Alternatively, if the UE is in idle mode, it can perform a reselection to
a cell on another frequency or RAT, based on the low value of the macro
signal level. This is also an undesirable outcome, as the UE can remain
associated with the macro cell in idle mode.

[0049] Problems related to paging channel reception by UEs in
heterogeneous networks are illustrated in FIG. 3. The paging signal can
comprise two components as described below.

[0050] A control channel signal indicating the Resource Allocation (RA)
corresponding to the data channel carrying the paging message. In 3GPP
LTE, the control channel can be a physical downlink control channel
(PDCCH) and the data channel can be a physical downlink shared channel
(PDSCH). Furthermore, a specific control channel format can be used for
signaling a data channel carrying the paging message. For example, a
PDCCH with a Downlink Control Information (DCI) format 1A or 1C as per
specifications TS 36.212 and TS 36.213 may be used for indicating a PDSCH
carrying a paging message. The DCI is convolutionally coded and the
codeword is scrambled with Paging Radio Network Transaction Identifier
(P-RNTI) prior to transmission. The paging message can include
information indicating a page for one or more UEs and can also include an
indication that a change of broadcast system information of the base
station is impending.

[0051] The paging signal can be transmitted only during a pre-determined
set of subframes. Based on its UE identifier, a UE determines a paging
subframe using a specified formula, during which it can receive paging
signals. The subframe determined based on the UE identifier is referred
to as the UE's paging occasion or the UE's paging subframe. Details of
determining the paging occasion for LTE UEs are specified in TS 36.304.
This mechanism enables the paging load to be distributed across the
predetermined set of subframes used for paging, while still ensuring that
the base station and the UE have a singular understanding of the UE's
paging occasion.

[0052] A UE may fail to decode the paging signal in the following two
scenarios:

[0053] A UE cannot successfully decode the DCI embedded in the PDCCH
signal and therefore, fails to determine that there is a PDSCH
transmission associated with the paging signal.

[0054] A UE successfully decodes DCI and determines the resource
allocation for the PDSCH, but it fails to decode the Transport Blocks
(TB) in the PDSCH transmission.

[0055] Both of these events lead to a paging failure. If the paging eNB
does not receive a paging response message from the UE within a certain
duration of the time following the paging signal transmission, the eNB
may re-page the UE by a re-transmission of the paging signal in the next
PO. If the UE cannot decode the paging signal successfully after several
paging attempts, this may lead to a severe paging failure as the eNB may
abandon further paging attempts. In a heterogeneous network, such paging
failures are likely due to interfering transmissions from neighboring
base stations. That is, if the UE is associated with a first cell when a
second cell is a strong interferer, transmissions from the second cell
can cause the UE to be unable to receive its paging signals. The UE is
then said to be in a paging outage condition.

[0056] In LTE, the predetermined set of subframes used for paging
transmission (the paging subframes of the cell) are restricted to
subframes 0, 4, 5 and 9 in FDD and subframes 0, 1, 5 and 6 in TDD. It may
be possible to ensure that the paging subframes of one cell coincide with
AB subframes of a neighbor cell that may pose interference problem. For
example, a femto cell in a TDD network may configure subframes 0, 1, 5
and 6 to be AB subframes. However, even in this case, it may not be
possible to avoid the neighbor cell signal transmissions. This is because
the (1) the neighbor cell transmits CRS during AB subframes, and (2) the
neighbor cell transmits criticially important signals such as PSS, SSS,
PBCH, SIB1, paging signals, PRS and CSI-RS during AB subframes. Note that
the SIB1 signal includes a PDCCH component and a PDSCH component;
consequently the PDCCH component of the SIB1 signal from a neighbor cell
can interfere with a PDCCH component of the paging signal from the
serving cell to a UE (as shown in FIG. 3), resulting in a paging failure.

[0057] Problems related to interference from CRS transmissions and
interference to CRS transmissions are illustrated in FIG. 4. The
following interference scenarios must be considered in a heterogeneous
network.

[0058] Neighbor cell CRS interference to serving cell CRS

[0059] Neighbor cell CRS interference to serving cell PDCCH

[0060] Neighbor cell PDCCH interference to serving cell PDCCH

[0061] Neighbor cell PDCCH interference to serving cell PDSCH

[0062] Neighbor cell PDSCH interference to serving cell PDCCH

[0063] Neighbor cell PDSCH interference to serving cell PDSCH.

[0064] A first cell and a neighbor cell of the first cell can select PCID
such that the CRS resource elements are substantially non-ovelapping.
This PCID planning where serving cell and neighbor cell use substantially
different CRS frequency offsets leading to non-overlapping CRS can
mitigate problem (i) above. However, this scheme can lead to problem (ii)
which cannot be avoided. Also problem (iii) is not possible to avoid due
to the dependence of codeword to RE mapping on the sub-block interleaver
and PCID. By configuring the number of symbols used for control channel
transmissions of the neighbor cell to be smaller than the number of
symbols used for control channel transmissions of the serving cell, it is
possible to reduce the impact of (iv). However, such an approach may be
difficult to use in a heterogeneous network comprising macro cells, pico
cells and femto cells. Moreover, such a restriction leads to not being
able to avoid problem (v) (e.g., SIB1 transmission from a femto cell can
interfere with macro cell's PDCCH in a paging subframe). Problem (vi) can
be avoided by frequency domain orthogonalization where the serving and
neighbor cells use non-overlapping RBs and this can be achieved by
network planning.

[0065] In summary, interference mitigation methods at the very least must
address problems (i), (ii), (iii) and (iv).

[0066] In particular, for TDD deployments and synchronous FDD deployments,
the frame time is aligned for all base stations within a geographical
area. If a macro UE roams close to a CSG femto cell, femto cell's
PDCCH/PDSCH associated with SIB1 in subframe 5 can interfere with paging
messages for such macro UEs. A similar problem can arise for a pico UE in
the range expansion area of a pico cell, due to the macro cell's SIB1
transmission. The interference can be large enough to result in increased
paging failures or to result in a paging outage.

[0067] FIG. 5 illustrates problems related to overlap of PBCH
transmissions from neighboring cells. The PBCH delivers the Master
information block (MIB), which is a fundamental component of the cell's
broadcast system information. The MIB indicates essential information for
system operation (such a operating bandwidth, system frame number, number
of antennas used, etc). The UE needs to successfully decode the PBCH and
use the information contained in the MIB to receive other parts of the
system information such as SIB1, SIB2 (system information block 2) etc. A
UE is expected to maintain up-to-date system information of a serving
cell. Changes in broadcast system information of the cell are indicated
in paging messages, wherein an indication that a system information
change is impending is transmitted. Upon receiving an indication that a
system information change is impending, the UE decodes the PBCH in a
predefined time interval and then goes on to receive other system
information. The PBCH is transmitted using fixed resources. In LTE, the
PBCH is transmitted in the center 6 resource blocks of every subframe 0.

[0068] In a heterogeneous network, since cells operating on a frequency
are synchronized, PBCHs of neighbor cells can overlap. This can lead to
UEs being unable to decode the PBCH and being unable to maintain
up-to-date system information. For example, if a macro UE is under the
coverage of a non-allowed femto cell, the PBCH transmissions of the
non-allowed femto cell can overlap the PBCH transmissions of a macro cell
the UE is associated with. The UE can then be unable to decode the PBCH
of the macro cell. A similar problem can occur when a pico UE is in the
range expansion area of a pico cell the UE is associated with, and also
in the coverage of a macro cell. In this situation, the UE can be unable
to receive the PBCH of the pico cell. In an FDD system, a time offset can
be applied by some cells on a frequency while still maintaining time
synchronization and alignment of subframe boundaries across all cells on
the frequency. Such a time offset is referred to as a subframe offset. A
subframe offset can avoid the problem of overlapping PBCH transmissions
between different cells on a frequency. However, a subframe offset cannot
be applied in TDD systems, due to rigidly defined patterns of subframes
that are used for uplink and downlink transmissions.

[0069] Problems related to the overlap of PSS and SSS transmissions from
neighbor cells in a frequency can also result in significant problems for
UEs in idle mode. As in the case of the PBCH, the PSS and the SSS are
transmitted using predefined resources. In an FDD LTE system, the PSS is
transmitted in the last symbol in slots 0 and 10 and in a TDD LTE system
it is transmitted in the 3rd symbol in subframes 1 and 6. In an LTE FDD
system, the SSS is transmitted two symbols before the last symbol in
slots 0 and 10 and in a TDD LTE system it is transmitted the penultimate
symbol in slots 1 and 11. The PSS and the SSS are used by UEs to remain
synchronized to the serving cell and to identify cells. The PSS and SSS
together indicate the PCID. Therefore being able to reliably receive the
PSS and the SSS is crucial to proper system operation.

[0070] If a UE is in the coverage of a femto cell, the PSS and SSS
transmissions from the femto cell can interfere with the PSS and SSS
transmissions of a macro cell operating on the same frequency.
Consequently, the UE may be unable to remain synchronized to the macro
cell. This can result in service outage, paging failures and other
undesirable consequences. Similar problems can occur when a pico UE
associated with a pico cell is in the range expansion area of the pico
cell. As in the PBCH case, a subframe offset can avoid the problem of
overlapping PSS and SSS transmissions between different cells on a
frequency. However, a subframe offset cannot be applied in TDD systems,
due to rigidly defined patterns of subframes that are used for uplink and
downlink transmissions.

[0071] Several embodiments are described to address the problems described
above.

[0072] According to a first embodiment of the invention, illustrated in
FIG. 6, the paging occasion of a UE can be changed if the UE experiences
significant interference in the normal paging occasion. For example, a UE
can have a paging occasion in a first subframe. The subframe that
corresponds to the paging occasion is typically predetermined. For
example, in LTE the subframe corresponding to the paging occasion is
determined as a function of an identifier or the UE. The UE may be camped
on a macro cell but be in the coverage of a non-allowed femto cell. In
such a situation, the UE may experience interference during its normal
paging occasion due to transmissions from the femto cell, and be unable
to receive paging messages. Upon determining that it can experience
interference during its paging occasion, the UE can change its paging
occasion to a new subframe. The new subframe for the paging occasion can
be a predetermined time offset later than the subframe corresponding to
the normal paging occasion. The new subframe for the paging occasion can
be chosen so that the likelihood of experiencing interference from the
femto cell in the new subframe is low. This is illustrated in FIG. 6A.

[0073] In the base station process of FIG. 6B, at 602, the base station
transmits a paging message for the UE in the regular paging occasion. At
604, if a response to the paging message is received by the bas station,
the procedure ends at 606. If no response is received, then at 608 the
base station pages the UE in the subframe that is a determined time
offset relative to the regular paging occasion. In the UE process of FIG.
6C, at 612, the UE determines that it is experiencing strong
interference. At 614, the UE determines whether interference to reception
of paging messages is likely. If not, the procedure ends at 616. If
interference of the reception of paging messages is likely, at 618, the
UE monitors for paging in the a subframe that is at a predetermined time
offset relative to the regular paging occasion.

[0074] Upon determining that a page message needs to be transmitted to the
UE, the base station can first transmit the page message in the subframe
corresponding to the UE's normal paging occasion. If the base station
does not receive a response to the page message from the UE, the base
station can transmit the page message in the subframe corresponding to a
new paging occasion. The new paging occasion can be a predetermined time
offset later than the subframe corresponding to the normal paging
occasion.

[0075] The determination by the UE that it can experience interference
during its paging occasion can be performed by determining that its
paging occasion coincides with a subframe during which a signal that can
interfere, such as SIB1 may be transmitted. Alternatively, the UE can
perform measurements during its paging subframe and determine that the
interference during the paging subframe is unacceptably high.

[0076] Furthermore, the choice of the new paging occasion can be such that
a subframe with specific characteristics is chosen for the new paging
occasion. For example, a macro UE may be associated with a macro cell
whose coverage overlaps the coverage of one or more femto cells. The
macro UE can be configured to choose a new paging subframe that is at
least a predetermined time offset later than the normal paging occasion,
and corresponds to the first AB subframe of the femto cell after the
predetermined time offset, if the macro UE is in the coverage of a
non-allowed femto cell. In another example, the macro UE can be
configured to choose a new paging subframe that is at least a
predetermined time offset later than the normal paging occasion, and
corresponds to the first AB subframe of the femto cell after the
predetermined time offset, wherein the femto cell does not transmit SIB1
in the first AB subframe of the femto cell, if the macro UE is in the
coverage of a non-allowed femto cell. Based on the rule specified, the
macro cell can uniquely determine the new paging occasion of the UE.

[0077] Furthermore, UEs that are implemented according to a legacy
specification (such as LTE Release 8 and Release 9 UEs) can be paged on
the normal paging occasions. On the other hand, UEs that are implemented
according to a newer specification (such as LTE Release 10 UEs) can be
paged on both their normal paging occasions and the new paging occasions.
Furthermore, in order to ensure that interference to the paging signals
are minimized, the new paging occasion can correspond to an AB subframe
of one or more neighbor cells. If a UE determines that its original
paging occasion coincides with subframe 5 of an interfering cell, and it
determines that the new paging occasion coincides with a CRS-only AB
subframe, it may decide to change its paging occasion to the new paging
occasion.

[0078] According to another embodiment, a UE can avoid a paging outage
scenario if interference to its paging signal is very likely. For
example, a macro UE may be associated with a macro cell and be in the
coverage of a non-allowed femto cell. If the UE determines that its
paging occasion overlaps, all or most of the time, a signal from the
femto cell, the UE can perform an inter-frequency or inter-RAT
reselection. Specifically, in LTE, if the paging occasion of the UE
overlaps a subframe 5 in an even numbered radio frame of the femto cell,
any paging signal from the macro cell to the UE will be interfered by
SIB1 transmissions from the femto cell. Consequently, the UE can perform
an inter-frequency or inter-RAT reselection if the paging occasion of the
UE overlaps a subframe 5 in an even numbered radio frame of the femto
cell. As a further simplification, and to ensure that the UE does not
need to first determine the system frame number of radio frames of the
femto cell, the UE can perform an inter-frequency or inter-RAT
reselection if the paging occasion of the UE overlaps any subframe 5 of
the femto cell.

[0079] According to another embodiment, the interference caused by an
interfering cell to the paging signals can be substantially reduced by
adjusting the number of symbols used for the control channel
transmissions. The interference to the PDCCH component of the paging
signal may be more significant than the interference to the PDSCH
component of the paging signal. Furthermore, the interference to the
PDCCH component of the paging signal is likely to be from the PDCCH
component of another signal, such as SIB1. In LTE the number of control
channel symbols can be semi-statically configured to 1, 2 or 3. In the
presence of interfering neighbor cells, a cell can always use a value of
3 for the number of control channel symbols in paging subframes that can
experience interference. That is, the semi-statically configured value of
the number of control channel symbols can be overridden for paging
subframes that can experience interference, and a value of 3 can be used.
Using the largest possible number of symbols for the control channel
transmissions ensures that the interference to the control channel is
minimized. For example, a macro UE may be in the coverage of a
non-allowed femto cell and be associated with a macro cell. The UE may
experience interference from the femto cell during its paging subframe.
The macro cell can override the semi-statically configured value of the
number of control channel symbols and use a value of 3 for the number of
control channel symbols in some or all of the paging subframe. The femto
cell can use a value of 1 for the number of control channel symbols in
the subframes that correspond to the paging subframe of the macro cell.
The number of control channel symbols is a function of the load in the
cell. Femto cells are generally lightly loaded and a smaller number of
control channel symbols may be adequate for control channel transmissions
in a femto cell. Thus, the interference experienced by the UE in the
PDCCH component of the paging signal is restricted to a single subframe,
increasing the likelihood of correctly decoding the PDCCH component of
the paging signal. Furthermore, the femto cell can use a low value of the
number of control channel symbols in all subframes, based on the coverage
of the femto cell overlapping the coverage of a macro cell.

[0080] In some embodiments, details of the new paging occasion can be
broadcasted by the network as part of system information. System
information is typically signaled in the MIB or one of the SIBs. Details
of the new paging occasion can comprise a frame index or sub frame index
or a system frame number. In embodiments where the new paging occasion is
using a time offset later than the subframe corresponding to the normal
paging occasion, the time offset value can also be broadcasted by the
network. In some embodiments, the specific PDSCH resource allocation
(resource block indices, Modulation and coding scheme) for the paging
message in the new paging location can also be broadcasted by the
network. Alternately, the specific PDSCH resource allocation for the
paging message in the new paging location can be based on pre-specified
values known a priori to the base station and the UE. In embodiments
where the specific resource allocation for the paging message in the new
paging location is either broadcasted or known a priori, the UE can read
the paging message directly on the PDSCH without decoding PDCCH in the
new paging location.

[0081] The UE can receive the PDCCH component of the paging signal in a
first subframe. The UE may be unable to decode the PDSCH component of the
paging signal in the first subframe. The UE can the attempt to decode the
PDSCH component of the paging signal in a second subframe, without
attempting to receive a PDCCH component of the paging signal in the
second subframe. For example, the first subframe can have little or no
interference in the symbols used for control channel transmissions, but
can have significant interference in the symbols used for PDSCH
transmissions. Thus, the UE may successfully decode the PDCCH component
of the paging signal, but be unable to decode the PDSCH component of the
paging signal, in the first subframe. If the UE is unable to decode the
PDSCH component of the paging signal in the first subframe, it can
monitor the second subframe for the PDSCH component of the paging signal.
The second subframe can overlap an AB subframe of a neighbor cell.
Alternatively, the UE can experience significant interference in the
symbols used for control channel transmissions in its normal paging
subframe. Therefore the UE can monitor an alternate subframe for the
PDCCH component of the paging channel and its normal paging subframe for
the PDSCH component of the paging subframe.

[0082] According to a second embodiment, the UE can modify its cell
reselection behavior based on whether overlap of resources reserved for
CRS transmission can occur. In a first approach illustrated in the
process 700 FIG. 7A, at 710, the UE may be camped on a macro cell but be
in the coverage of a non-allowed femto cell. In such a situation, the
resource elements used by the macro cell and the femto cell for their
respective CRS transmissions can overlap, resulting in the UE being
unable to perform correct measurements of both the macro cell and the
femto cell and possibly other cells. For example, the PCID of the macro
cell and the femto cell can be such that the resource elements used for
their respective CRS transmissions overlap. At 720, the UE determines
whether the CRS transmissions of the macro cell and the femto cell can
overlap. This determination can be done by (a) first detecting the PCID
of the femto cell, (b) then, based on the PCID, determining the resource
elements used for CRS transmissions of the femto cell, and (c) comparing
the resource elements used for CRS transmissions of the femto cell and
the resource elements used for CRS transmissions of the macro cell.

[0083] In addition to the PCID, information pertaining to the number of
CRS transmission ports in the neighbor cell and the serving cell can be
used. The UE can determine the number of CRS transmission ports of the
serving cell based on PBCH decoding. On the other hand, the number of CRS
transmission ports for the neighbor cell can be determined based on
either neighbor cell PBCH decoding or by means of assistance data
signaled by the serving cell that includes this information. When the
number of CRS transmission ports for the serving and neighbor cells are
different, different situations may arise.

[0085] When both the serving cell and the neighbor cell have 2 Tx, CRS
collision occurs when mod(PCIDserving, 3)=mod(PCIDneighbor, 3).

[0086] When the serving cell has 4 Tx and neighbor cell has 2 Tx, CRS
ports #2 and #3 for the serving cell will not experience any CRS
interference from the neighbor cells as ports #0 and #1 are mapped to a
different set of OFDM symbols relative to ports #2 and #3.

[0087] Two reselection thresholds may be configured in the UE by the
network (e.g., by signaling in SIB or in a RRC message as part of RRC
connection release), one applicable to the case when the CRS
transmissions of a femto cell overlaps the CRS transmissions of a macro
cell, and another applicable to the case when the CRS transmissions of
the femto cell does not overlap the CRS transmissions of the macro cell.
A macro UE with CRS Interference Cancellation (IC) or Interference
Rejection (IR) receiver capabilities may be able to stay on the same
frequency even when the interference from the femto cell is large, if
there is no CRS collision. The UE may be able to remain attached to and
remain schedulable by the macro cell even when the RSRP difference
between the serving cell and the neighbor cell is as low as, say, -20 dB.
However, CRS interference rejection/cancellation capabilities may be
limited when there is CRS collision. The UE may be able to remain
attached to the macro cell only up to, say, -6 dB in this case.
Therefore, different reselection thresholds matched to the receiver
capabilities in the non-colliding and colliding CRS cases may be
necessary.

[0088] In FIG. 7A, at decision block 730, if there is no overlap of
resource elements used for CRS, then the UE follows normal idle mode
relesction procedures. If overlap occurs, at 740 the UE determines
whether the neighbor cell signal level is greater than the serving cell
signal level plus a threshold. At 742, if the neighbor cell signal level
is not greater than the serving cell signal level plus a threshold, the
UE follows normal idle mode reselection procedures. At 750, if the
neighbor cell signal level is greater than the serving cell signal level
plus the threshold, the UE performs inter-frequency or inter-RAT
reselection.

[0089] If the UE determines that the resource elements used for the CRS
transmissions of the macro cell and the femto cell overlap, it can
perform an inter-frequency reselection or an inter-RAT reselection.
According to a further embodiment, the UE can perform the inter-frequency
or inter-RAT reselection only if a signal level of the femto cell is no
less than a signal level of the macro cell plus a threshold. The signal
level metric used to determine whether the signal level of the femto cell
is no less than the signal level of the macro cell plus a threshold can
be obtained by measurements of resources other than the CRS.

[0090] According to another embodiment illustrated in FIG. 7A at 734 the
UE applies different reselection biases for the serving frequency or
serving cell based on whether overlap occurs. In one implementation, if
the UE determines that the resource elements used for the CRS
transmissions of the macro cell and the femto cell overlap, it can apply
a negative bias to the serving frequency. The negative bias can result in
cells on another frequency or another RAT being viewed by the UE as
suitable candidates for reselection, and the UE can perform a
inter-frequency or inter-RAT reselection. If the UE determines that the
resource elements used for the CRS transmissions of the macro cell and
the femto cell do not overlap, it can apply a positive bias to the
serving frequency. The positive bias can ensure that the UE remains
camped on the macro cell even if a signal level of the femto cell (such
as RSRP) is higher than that of the macro cell. Additionally, or
alternatively, the UE can apply a negative bias to the serving cell if
the UE determines that the CRS transmissions of the macro cell and the
femto cell overlap substantially. The UE can apply a positive bias to the
serving cell if the UE determines that the CRS transmissions of the macro
cell and the femto cell do not overlap substantially.

[0091] In a second approach illustrated in the process 701 of FIG. 7B, at
711, a femto cell can determine that its coverage overlaps the coverage
of one or more neighbor cells. At 721, the femto cell can determine the
resource elements used by the one or more neighbor cells. At 731, if the
femto cell determines that the resource elements used for CRS
transmissions by at least one of the one or more neighbor cells overlaps
the resource elements used for the femto cell's CRS transmissions, the
femto cell can use a different set of resource elements for its CRS
transmissions. The femto cell can use the different set of resource
elements for its CRS transmissions in some or all subframes. For example,
741, the femto cell can use the different set of resource elements for
its CRS transmissions only during its AB subframes. Additionally, the
different set of resource elements for the femto cell's CRS transmissions
can be obtained by applying offsets in time and frequency to the original
resource elements, as shown in FIG. 7. The offsets in time and frequency
can be signaled to UEs in the network, for example by macro cells.

[0092] According to another embodiment, a femto cell can modify access
restrictions based on whether its CRS transmissions substantially overlap
the CRS transmissions of a macro cell. For example, a femto cell may be a
CSG cell and allow access only to a certain group of users. The femto
cell can determine that its coverage may overlap one or more macro cells.
The femto cell can further determine that the resource elements it uses
for transmissions of CRS can substantially overlap the resource elements
used for CRS transmissions by at least one of the one or more macro
cells. The femto cell can then modify access restrictions such that all
users can access the femto cell. The modifying of access restrictions can
ensure that a UE does not remain in the coverage of the femto cell
without being able to connect to the femto cell. That is, if all femto
cells in the network perform such a procedure, UEs do not encounter
non-allowed femto cells whose CRS transmissions overlap the CRS
transmissions of macro cells. The modification of access restrictions can
be performed by changing the status of the femto cell from a CSG cell to
a "hybrid access" cell or "open access" cell.

[0093] According to a third embodiment, a base station can use alternate
resources to transmit PBCH contents or PBCH related information. The
alternate resources used to transmit PBCH contents can be predetermined
so that UEs can receive the PBCH contents or the PBCH related information
in these resources. In a first approach illustrated in FIG. 8A, at 810, a
base station determines that its system information needs to be changed.
At 820, the base station determines whether its coverage overlaps the
coverage of one or more neighbor cells, such as femto cells. If not, at
822, the base station resumes normal operation. At 830, in the presence
of overlap, the base station transmits a system information change
indication message, wherein the message includes the PBCH contents. UEs
that are associated with the base station (in connected mode or idle
mode) and are in the coverage of non-allowed femto cells can receive the
system information change indication message and the included PBCH
contents. This can enable the UEs to receive other system information
such as system information block 1 (SIB1). Alternatively, at 840, the
base station can transmit a system information indication message,
wherein the message includes an indication of alternate resources uses
for PBCH transmission. UEs that are associated with the base station (in
connected mode or idle mode) and are in the coverage of non-allowed femto
cells can receive the system information change indication message and
the indication of alternate resources for PBCH transmission. The UEs can
then decode the PBCH in the alternate resources. This can enable the UEs
to receive other system information.

[0094] The PBCH contents transmitted in the paging message can include one
or more of the information elements in the MIB. For example, the paging
message can include one or more of the downlink bandwidth, information
related to the system frame number and information related to the
physical HARQ indicator channel (PHICH).

[0095] According to another embodiment, the UE can receive the PBCH of a
macro cell and a femto cell by transmitting the PBCH of the femto cell in
alternate resource blocks in a predetermined manner. For example, a macro
UE may be associated with a macro cell. In a second approach illustrated
in the process 801 of FIG. 8B, at 811, the UE decodes the PBCH of the
macro cell in a first set of resource blocks of the macro cell. The UE
can receive additional system information of the macro cell after
decoding the PBCH. At 821, the UE receives a `Femto cell PBCH resource
offset` parameter from the macro cell. At 831, the UE detects a physical
cell identifier of a femto cell. The UE then roam into the coverage of a
femto cell and detect the PCID of the femto cell. At 841, the UE
determines a second set of resource blocks by applying an offset equal to
the `Femto cell PBCH resource offset` to the first set of resource
blocks. At 851, the UE attempts to decode the PBCH of the femto cell in
the resource blocks of the femto cell that overlap the second set of
resource blocks. The UE can continue to decode the PBCH of the macro cell
in the first set of resource blocks as needed.

[0096] The femto cell may be configured to transmit its PBCH using a
normal set of resource blocks. In process 802 of FIG. 8c, at 811, the
femto cell determines that its coverage may overlap the coverage of a
macro cell. At 822, the femto cell transmits its PBCH in an alternate set
of resource blocks. The alternate set of resource blocks can be offset
from the normal set of resource blocks by an amount equal to the `Femto
cell PBCH resource offset`. It should be noted that the procedure can be
applied to any combinations of cells instead of the macro cell and femto
cell combination described above. The macro cell can indicate a PBCH
resource offset for any specific cell or set of cells (for example as
part of a neighbor list). When a PCID of a neighbor cell is detected, the
UE can apply the corresponding PBCH resource offset (if indicated), to
obtain the alternate resource blocks used by the neighbor cell for PBCH
transmission.

[0097] In synchronous networks, interference PSS transmissions of two
cells on the same frequency can interfere with each other and SSS
transmissions of two cells on the same frequencies can interfere with
each other. The following solutions mitigate this problem.

[0098] The PSS and/or the SSS can be transmitted using alternate resource
elements. The alternate resource elements can be protected from
interference. Furthermore, the PSS and/or the SSS can be transmitted
using both the resource elements used normally for PSS and/or SSS
transmission and the alternate resource elements. For example, a macro UE
may be in the coverage of a non-allowed femto cell, and thus may be
unable to receive the PSS and/or SSS of the macro cell. To overcome such
a problem, the femto cell can first recognize that its coverage overlaps
that of the macro cell. The femto cell can then transmit the PSS and/or
the SSS in alternate resource elements. The femto cell can choose the
alternate resource elements such that the alternate resource elements do
not overlap some critically important transmissions from the macro cell.
Furthermore, the femto cell can also ensure that some or all of the
resource elements that overlap the PSS and/or the SSS transmissions of
the macro cell do not carry any transmissions from the femto cell.

[0099] The alternate resource elements used for PSS and/or SSS
transmissions can be offset by a time duration from the normal resource
elements used for PSS and/or SSS transmissions. Preferentially, the
offset can be a certain number of subframes. That is, the PSS and/or SSS
can be transmitted in alternate subframes but in the same OFDM symbol.
For example, the PSS may normally be transmitted in the 3rd symbol
of subframe 1 and the 3rd symbol of subframe 6; and the SSS may
normally be transmitted in the 13th symbol in subframe 1 and the 13th
symbol in subframe 6. The femto cell can instead transmit the PSS in the
3rd symbol of subframe 3 and the 3rd symbol of subframe 8.

[0100] A UE that receives the PSS and/or SSS from the femto cell may be
unaware that the femto cell is using alternate resources for PSS and/or
SSS transmissions. Hence the UE can interpret the frame timing of the
femto cell assuming the PSS and/or SSS are being transmitted in the
normal resource elements. Such a UE may be either a macro UE that is in
the coverage of the femto cell, a associated with the femto cell, or a UE
attempting to associate with the femto cell. Thus, in the above example,
such a UE can assume that the subframes in which the PSS transmissions
are received are subframe 1 and 6, and the subframes in which the SSS
transmissions are received are subframe 1 and 6. The UE will then be
unable to decode the PBCH or critical transmissions such as SIB1, paging
etc from the femto cell. In order to prevent such problems, the time
duration offsets used for the PSS and/or the SSS can be made known to the
UE apriori. For example, a macro cell can signal the time duration offset
to the UEs. The time duration offset can also be a fixed value for a
class of cells (such as femto cells or CSG cells) and may not need to be
signaled. The UEs can use the time duration offset to correct their
interpretation of the frame boundary of the femto cell. Thus, in the
above example, a time duration offset equal to two subframes, for the
femto cell, is made known apriori to the UE. Based on this
interpretation, the UE can determine that the subframes in which the PSS
transmissions are performed by the femto cell are subframes 3 and 8, and
the subframes in which the SSS transmissions are performed by the UE are
subframes 3 and 8. Thus, the functions and apparatus in the UE related to
PSS/SSS reception (such as cell search) can have a first interpretation
of the frame timing; and the other functions and apparatus in the UE (for
example, other physical layer functions, medium access control functions
and measurements related functions) can have a second interpretation of
frame timing. The second interpretation of frame timing can be an offset
by a time period from the first interpretation of frame timing.
Furthermore, it should be noted that such an approach can be used to
avoid overlap of PBCH transmissions of neighbor cells.

[0101] According to another embodiment, if a macro UE determines that PSS
and/or SSS of a non-allowed femto cell overlaps the PSS and/or SSS
transmissions of a serving macro cell, the UE can perform an
inter-frequency or inter-RAT reselection. Thus, the UE can remain
associated with the macro cell even if the non-allowed femto is a strong
interfering cell, unless the PSS and/or SSS transmission of the serving
macro cell overlap the PSS and/or SSS transmissions of the non-allowed
femto cell. Similarly, a pico UE in the range expansion area of the pico
cell may experience high P/S-SCH interference due to interference from a
macro cell. Such a UE can perform inter-frequency or inter-RAT
reselection if the PSS and/or SSS of the macro cell and the pico cell
overlap.

[0102] According to another embodiment, a UE can recognize its proximity
to a femto cell and determine that it needs to attempt reception of PSS
and/or SSS in alternate resource elements. For example, a UE may
determine that it is close to a femto cell based on RRM measurements on
the frequency. RRM measurements (such as RSRP) can indicate that the UE
is close to the femto cell. The UE can also determine that the coverage
of the femto cell overlaps the coverage of a macro cell. The UE can then
attempt to receive the PSS and/or SSS in the alternate resource elements.
The UE can apply such a procedure when it roams into the coverage of a
femto cell whose coverage overlaps that of a macro cell. The UE can also
apply such a procedure when it is powered up in the coverage of a femto
cell. Other means of recognizing proximity to a femto cell can be used,
including positioning methods such as global positioning system (GPS) and
Enhanced observed time difference (E-OTD).

[0103] In the foregoing specification, specific embodiments of the present
invention have been described. However, one of ordinary skill in the art
will appreciate that various modifications and changes can be made
without departing from the scope of the present invention as set forth in
the claims below. Accordingly, the specification and figures are to be
regarded in an illustrative rather than a restrictive sense, and all such
modifications are intended to be included within the scope of present
invention. The benefits, advantages, solutions to problems, and any
element(s) that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as critical, required, or
essential features or elements of any or all the claims. The invention is
defined solely by the appended claims including any amendments made
during the pendency of this application and all equivalents of those
claims as issued.